1. The Field of the Invention
The present invention relates generally to thermoplastic films. Specifically, the invention relates to stretched thermoplastic films, and to methods of stretching thermoplastic films.
2. Background and Relevant Art
Thermoplastic films are a common component in various commercial and consumer products. For example, grocery bags, trash bags, sacks, and packaging materials are products that are commonly made from thermoplastic films. Additionally, feminine hygiene products, baby diapers, adult incontinence products, and many other products include thermoplastic films to one extent or another.
The cost to produce products including thermoplastic film is directly related to the cost of the thermoplastic film. Recently the cost of thermoplastic materials has risen. In response, many manufacturers attempt to reduce the amount of thermoplastic film in a given product in order to reduce production costs.
One way manufacturers may attempt to reduce production costs is to stretch the thermoplastic film, thereby increasing its surface area and reducing the amount of thermoplastic film needed to produce a product of a given size. Common directions of stretching include “machine direction” and “transverse direction” stretching. As used herein, the term “machine direction” or “MD” refers to the direction along the length of the film, or in other words, the direction of the film as the film is formed during extrusion and/or coating. As used herein, the term “transverse direction” or “TD” refers to the direction across the film or perpendicular to the machine direction.
Common ways of stretching film in the machine direction include machine direction orientation (“MDO”) and incremental stretching. MDO involves stretching the film between pairs of smooth rollers. Commonly, MDO involves running a film through the nips of sequential pairs of smooth rollers. The first pair of rollers rotates at a speed less than that of the second pair of rollers. The difference in speed of rotation of the pairs of rollers can cause the film between the pairs of rollers to stretch. The ratio of the roller speeds will roughly determine the amount that the film is stretched. For example, if the first pair of rollers is rotating at 100 feet per minute (“fpm”) and the second pair of rollers is rotating at 500 fpm, the rollers will stretch the film to roughly five times its original length. MDO stretches the film continuously in the machine direction and is often used to create an oriented film.
Incremental stretching of thermoplastic film, on the other hand, typically involves running the film between grooved or toothed rollers. The grooves or teeth on the rollers intermesh and stretch the film as the film passes between the rollers. Incremental stretching can stretch a film in many small increments that are evenly spaced across the film. The depth at which the intermeshing teeth engage can control the degree of stretching. Often, incremental stretching of films is referred to as ring rolling.
To MDO stretch a film, manufacturers commonly heat the film to an elevated temperature and stretch the film in the machine direction. Commonly, manufacturers will stretch the thermoplastic film between approximately 300 to 500 percent of the film's original length or more. Unfortunately, stretching thermoplastic films in the machine direction using conventional methods can weaken the film.
Thermoplastic films have a variety of different strength parameters. For example, the tensile strength of a thermoplastic film is the maximum stress that a film can withstand while being stretched before it fails. Tear resistance of a thermoplastic film is the amount of force required to propagate or enlarge a tear that has already been created in a film. Still further, impact resistance is the force required to puncture a film. Stretching thermoplastic films in the machine direction using conventional methods can reduce the film's machine-direction tear resistance and/or puncture resistance.
The weakening of a film upon stretching may dissuade manufacturers to stretch a film despite the potential material savings. For example, one common use of thermoplastic films is as bags for liners in trash or refuse receptacles. It is desirable to have trash bags with a high tear resistance to help prevent tearing of the trash bag and associated spilling of the contents during disposal thereof. Another common use of thermoplastic films is as flexible plastic bags for storing food items. Similar to trash bags, a high tear resistance in food storage bags can help prevent tearing of the bags and associated spilling or spoiling of food.
Accordingly, there are a number of considerations to be made in thermoplastic films and manufacturing methods.